1. Field of the Invention
The present invention relates to an active-matrix display apparatus employing pixel circuits each including a light emitting device and relates to a driving method provided for the display apparatus. In addition, the present invention also relates to electronic instruments each including the active-matrix display apparatus as the display unit thereof.
2. Description of the Related Art
In recent years, planar display apparatus of a self-light-emission type are developed intensively as well as extensively as display apparatus each employing organic EL (Electro Luminance) devices which each serve as a light emitting device. The organic EL light emitting device is a device taking advantage of a phenomenon in which light is emitted when an electric field is applied to an organic thin film employed in the device. Since the organic EL light emitting device can be driven to operate by applying a voltage not higher than 10 V, the organic EL light emitting device consumes little power. In addition, since the organic EL light emitting device is a device of a self-light emission type capable of emitting light by itself, the display apparatus employing the organic EL light emitting devices does not require an illumination member. Thus, the display apparatus employing the organic EL light emitting devices can be made light and thin with ease. On top of that, since the organic EL light emitting device is a very fast device having a response time of about several microseconds, the display apparatus employing the organic EL light emitting devices does not generate a residual image.
Planar display apparatus, which each employ pixel circuits each including an organic EL light emitting device and serve as a display apparatus of the self-light-emission type, include among others an active-matrix display apparatus employing pixel circuits each having thin-film transistors integrated therein to serve as an active device. In recent years, such active-matrix display apparatus are developed intensively as well as extensively. Active-matrix planar display apparatus of the self-light-emission type are described in documents as follows: Japanese Patent Laid-open Nos. 2003-255856, 2003-271095, 2004-133240, 2004-029791, 2004-093682, 2006-251322, and 2007-310311.
FIG. 29 is a model circuit diagram showing a typical example of the existing display apparatus of the active-matrix type. The display apparatus is configured to include a pixel array section 1 and driving sections surrounding the pixel array section 1. The driving sections are a horizontal selector 3 also referred to hereafter as a signal selector and a write scanner 4. The pixel array section 1 resembling a matrix of pixel circuits 2 has signal lines SL each laid as one of the columns of the matrix and scan lines WS each laid as one of the rows of the matrix. Each of the pixel circuits 2 is located at an intersection of one of the signal lines SL and one of the scan lines WS. In order to make the following explanation easy to understand, the diagram of FIG. 29 shows only one pixel circuit 2 at one intersection. The write scanner 4 has a shift register. The write scanner 4 operates in accordance with a clock signal ck received from an external source. The write scanner 4 also receives start pulses sp supplied by an external source sequentially. Receiving the clock signal ck and such start pulses sp, the write scanner 4 asserts control signals sequentially on the scan lines WS. The horizontal selector 3 asserts a video signal on the signal line SL with a timing adjusted to the row-after-row sequential scan operation carried out by the write scanner 4.
The pixel circuit 2 employs a signal sampling transistor T1, a device driving transistor T2, a signal holding capacitor C1 and a light emitting device EL. The device driving transistor T2 is a transistor of the P-channel type. A specific one of the two current terminals of the device driving transistor T2 serves as the source electrode of the device driving transistor T2. The specific current terminal serving as the source electrode is connected to a power-supply line. The other one of the two current terminals of the device driving transistor T2 serves as the drain electrode of the device driving transistor T2. The other current terminal serving as the drain electrode is connected to the anode electrode of the light emitting device EL. The gate electrode of the device driving transistor T2 is used as the control electrode of the device driving transistor T2. The gate electrode of the device driving transistor T2 is connected to the signal line SL through the signal sampling transistor T1. A control signal asserted on the scan line WS puts the signal sampling transistor T1 in a turned-on state. The signal sampling transistor T1 put in a turned-on state samples a video signal asserted by the horizontal selector 3 on the signal line SL and stores the video signal in the signal holding capacitor C1. The video signal stored in the signal holding capacitor C1 is applied to the gate electrode of the device driving transistor T2 as a gate-source voltage Vgs which drives the device driving transistor T2 to output a drain-source current Ids to the light emitting device EL. Thus, the light emitting device EL emits light having a luminance according to the video signal. The gate-source voltage Vgs represents an electric potential appearing on the gate electrode of the device driving transistor T2 as an electric potential taking an electric potential appearing on the source electrode of the device driving transistor T2 as a reference. On the other hand, the drain-source current Ids is a current flowing between the drain and source electrodes of the device driving transistor T2.
The device driving transistor T2 operates in a saturated region. The relation between the gate-source voltage Vgs and the drain-source current Ids is expressed by Eq. (1) given as follows:Ids=(½)μ(W/L)Cox(Vgs−Vth)2  (1)
In the above equation, reference notation μ denotes the mobility of the device driving transistor T2 whereas reference notation W denotes the width of the channel of the device driving transistor T2. Reference notation L denotes the length of the channel of the device driving transistor T2 whereas reference notation Cox denotes the gate insulation film capacitance per unit area of the device driving transistor T2. Reference notation Vth denotes the threshold voltage of the device driving transistor T2. As is obvious from the characteristic expressed by Eq. (1), when the device driving transistor T2 is operating in a saturated region, the device driving transistor T2 functions as a constant-current source supplying a drain-source current Ids according to the gate-source voltage Vgs to the light emitting device EL.
FIG. 30 is a diagram showing graphs each representing a relation between a voltage applied to the light emitting device EL and a driving current flowing through the light emitting device EL. That is to say, FIG. 30 is a diagram showing graphs each representing a voltage-to-current characteristic of the light emitting device EL. As is obvious from the above description, the driving current flowing through the light emitting device EL is the drain-source current Ids generated by the device driving transistor T2. The voltage applied to the light emitting device EL is a voltage V appearing on the anode electrode of the light emitting device EL. The horizontal axis represents the voltage V appearing on the anode electrode of the light emitting device EL whereas the vertical axis represents the drain-source current Ids which is also referred to hereafter as the driving current cited above. The voltage V appearing on the anode electrode of the light emitting device EL is the voltage appearing on the drain electrode of the device driving transistor T2. The voltage-to-current characteristic of the light emitting device EL changes with the lapse of time as indicated by a change from the solid-line curve to the dashed-line curve. To be more specific, as time lapses, the voltage-to-current characteristic of the light emitting device EL tends to be shifted to the right. Thus, even if the driving current Ids is held at a constant magnitude, the anode-electrode voltage V (or the drain voltage V) changes with the lapse of time. To be more specific, even if the driving current Ids is held at a constant magnitude, the anode-electrode voltage V (or the drain voltage V) increases. Fortunately, however, the device driving transistor T2 employed in the pixel circuit 2 shown in the diagram of FIG. 29 is operating in a saturated region to generate a drain-source current Ids dependent on the gate-source voltage Vgs without regard to changes in drain voltage V. Thus, the luminance of light generated by the light emitting device EL can be sustained at a fixed value independently of the changes of the voltage-to-current characteristic of the light emitting device EL with the lapse of time.
FIG. 31 is another model circuit diagram showing a typical example of the existing display apparatus of the active-matrix type. The pixel circuit 2 shown in the diagram of FIG. 31 is different from the pixel circuit 2 shown in the diagram of FIG. 29 in that, in the case of the pixel circuit 2 shown in the diagram of FIG. 31, the device driving transistor T2 is a transistor of the N-channel type in place of a transistor of the P-channel type as is the case with the pixel circuit shown in the diagram of FIG. 29. By employing transistors of the N-channel type as both the signal sampling transistor T1 and the device driving transistor T2, the process of manufacturing the pixel circuit 2 becomes easier to carry out in many cases.